Our efforts in the last year have concentrated on the microtubule severing enzyme spastin. Spastin is mutated in 40% of hereditary spastic paraplegias, a group of neurodegenerative disorders characterized by axonopathy. We have previously determined the X-ray structure of spastin and proposed a novel mechanism for microtubule severing (Roll-Mecak and Vale, 2008). We have made progress in the last year on Aim 1a of our research program that concentrates on understanding the biophysical mechanism of microtubule severing enzymes, specifically the interaction between spastin and the microtubule.